The present invention relates to a method for controlling and supervising a continuous or semi-continuous process comprising at least two sub-processes and in particular to an improved method for controlling and supervising a process for production of paper or board. The present invention also relates to a device for carrying-out the invented method.
Material producers, such as producers of pulp, paper and board, are facing a rapidly changing world. They must meet increasing customer demands for specific grades in smaller batches. The products must meet quality demands specified within narrow limits. The increased environmental awareness is met by large efforts not only to meet the more stringent demands and regulations set on their activity but also to minimize the impact of their activity on the environment. Increased recycling within the production process as well as an increased use of reclaimed matter as raw material or additions to the process also put higher demand on the control and supervision of the process. Additionally these producers face a tougher competition, which can only be met with high quality products.
The product quality is dependent on many parameters which are influenced by the raw material supplied to the process, especially the consistency of raw material supplied, the additions made during treatments and the production conditions used under these treatments. Thus, the product quality can change substantially throughout these processes. Therefore, a method for process control needs to employ a system with sensors or devices for sampling throughout the process line and means to execute measurements and sampling, means to collect, collate and process information obtained from the process and means to execute corrective actions in the process.
One such technique is pulp tracking, which is a method for supervision of the pulp process. In general terms a process for production of paper or board comprises two main sub-processes, namely a pulp process and a paper or board manufacturing sub-process. When pulp tracking a specific volume of pulp is followed throughout the pulp step. The pulp tracking function divides the pulp process flow into a plurality of imaginary slices, each slices typically representing 2-10 tons of pulp corresponding to 5-20 minutes of production. The values for production variables, such as temperature, mass and liquid flows, together with values for quality variables or characterizing variables in the form of test results from samples taken on fiber and liquid from the slice at this production unit, are measured, stored and labeled in a slice database. Pulp tracking and similar techniques provide on-line quality information to be used for corrective actions. It also provides product documentation.
Other techniques comprise software to control, supervise and execute measurement of process variables and sampling to determine quality parameters throughout a product-line, whether in a process for production of pulp, paper or board.
Production variables measured or otherwise determined are typically temperature and flow of mass and liquids in the pulp process, temperature, moisture, pH and web speed, motor effects and pressures in the paper and board processes.
Quality variables determined by tests and analysis of samples are typically kappa number, viscosity, strength parameters, brightness, and residual bleaching chemicals.
It is known to characterize the conditions of a complex process or to predict quality variables for the product downstream or for the final product using, algorithms, statistical models obtained from multivariate data-analysis such as Principal Component Analysis (PCA), Principal Component Regression (PCR), Partial Least Square Regression (PLS), MultiLinear Regression Analysis (MLR), Discriminant Analysis or in a neural network. Before the algorithms and models are used they need to be calibrated based on a comparison of measured or otherwise determined values of the variables to be used for the prediction with measured or otherwise determined values of the actual outcome for the variable to be predicted.
U.S. Pat. No. 5,402,333 discloses a system and method for improving model product property estimates. One or more process simulation models are run on a computer in parallel with the actual process to provide estimates of steam composition and/or product properties further down the line, which estimates are used to control the process. The system provides estimates for closed-loop control process, i.e. the measured variables used for the estimate are controlled based on the estimate. In this way the quality of the end product can be improved. However, this control system does not affect the present batch, i.e. the batch on which the estimates are based, but the next batch in line. This means that there might be a rejected batch before high quality batches are delivered. A rejected batch may also be the outcome every time the production conditions are altered. Thus, there is a need for a control system, which is able to control the process in such a way that the rejection from the process is kept to a minimum.
The paper xe2x80x9cControl of paper basis weight by expert SQC/SPC algorithm using distributed controlsxe2x80x9d, by M A Keyes and A Kaya, Automatic Control Word Congress 1990, pages 395-400, discloses a feedback control design and a statistical quality/process control (SQC/SPC) system. The process model in this system is used to predict the values of basis weight further down the line based on measured variable values earlier in the process. Based on such predictions of the outcome these earlier parameters are controlled, i.e. it is a feed back control system. This means that the same disadvantages apply to this control system as described above in conjunction with U.S. Pat. No. 5,402,333, namely only the next batch in line will be affected by the control.
The primary object of the present invention is to provide an improved method for process control for an essentially continuous process comprising at least two sub-processes, such as the production of paper or board, which minimizes the rejection for the produced product. This improved method shall also meet the increasing demands for higher quality, less impact on the environment, reduced consumption of energy and other resources and narrower specification limits at reduced or the same costs.
Another object of the present invention is to provide a method for simplifying the control and supervision of an essentially continuous process, comprising at least two sub-processes, in which the measured or otherwise determined values for a first sub-process are used to correct and/or predict the values in a subsequent second sub-process. It is further an object that a method according to the present invention shall include the correlation of the effects on a predicted variable of any corrective actions taken in the process.
Another object of the present invention is to provide a method, in which unstable or malfunctioning sensors are detected and identified. Thereby early warnings can be given such that the operator or the system for process control temporarily disregards values from this sensor in the waiting for a check and possible replacement of the sensor. The same applies for defective or malfunction production means.
According to the invention the above object is achieved by a method for control an essentially continuous process comprising at least two sub-processes, by tracking, processing and correcting variables for the product, the production means and/or any process media throughout the process line, wherein;
variable values from a first sub-process are measured,
samples from the first sub-process are taken and tested or analyzed to determine a variable value;
the flow of the first sub-process is divided into imaginary slices, each slice representing a specific volume of the first sub-process flow and that at least some variable values are related to the specific slice volume;
any variable value obtained by measurements, sampling, analyzing or testing is processed and any variable value obtained for a specific slice volume is related to that slice volume;
at least one multivariate model is developed based on the variable values for a multiple of slice volumes from the first sub-process,
the multivariate model is combined with a processed variable value, from the first sub-process, representing a specific slice volume,
a variable value and/or a quality variable for the specific slice volume and/or another slice volume for a subsequent second sub-process downstream is predicted based on the combination of the multivariate model and the processed variable value, and
the prediction is used for executing a corrective action on the subsequent second sub-process.
Thus, with the method according to the present invention it will be possible to correct a batch, which is already in production by using feed forward to the subsequent second sub-process. Thereby the rejection rate on produced batches will be reduced substantially. Also the quality of the produced batches can be kept in much narrower limits.
Other advantages of the present invention includes its capabilities to provide an improved documentation of production and quality variables as well as corrective actions taken throughout the continuous process which is representative also for smaller product quantities.
Other further advantages of the present invention will become apparent from the description of the invention and the preferred embodiments of the invention.